This invention relates to a liquid-level sensor and, more particularly, to the liquid-level sensor using optical fibers with surface discontinuities to indicate the presence of a liquid contacting each of the optical fibers.
Several techniques exist for measuring the level of a liquid. In the simplest, a mechanical dipstick provides a measure of the depth of the liquid relative to a fixed reference point, such as the bottom of a container. A mechanical float that rides on the surface of the liquid may be used to measure the liquid level relative to a float attachment point. The dipstick cannot be used for most remote sensing applications. The mechanical float is complex in practice and prone to failure in some applications.
Electrical techniques for measuring the liquid level are also available. In one widely used approach, a capacitor is formed by two facing metal plates and the liquid between them. As the liquid level varies, the capacitance changes. The relation between liquid level and capacitance may be calibrated, so that subsequent capacitance measurements may be related to liquid level. The capacitance technique has the virtues that there are no mechanical movements and there is an electrical readout that is suited for remote sensing applications. However, it requires relatively large components in the liquid container and also requires the application of a voltage across the plates and thence across the liquid. The latter requirement is associated with substantial power consumption and also may be unacceptable where the liquid is flammable.
Optical techniques are also known. In one, a light beam is directed from a source vertically through the container to a detector. The reduction in measured intensity of the light beam is a measure of the amount of liquid between the source and the detector, and thence the liquid level. This approach has an electrical output signal and no electrical potential is applied through the liquid. However, this method is limited as to the depth of liquid that may be measured due to the attenuation of the light in the liquid, and is also sensitive to influences such as vapor above the liquid, currents within the liquid, bubbles in the liquid, misalignment of the source and the detector, and the like.
Other optical techniques require that a relatively large-diameter optical conductor be immersed into the liquid, or that some portion of the instrumentation be immersed into the liquid or that the optical conductor be of sufficiently large size to carry the instrumentation. In other approaches, the light source and/or the light detector must be very precisely positioned relative to the optical conductor. These approaches all utilize electrical sensing instrumentation.
Although there are numerous techniques for measuring liquid level and other properties, there is a need for an improved approach which overcomes the difficulties discussed above with the existing approaches. The present invention fulfills this need, and further provides related advantages.
The present invention provides an apparatus and method for measuring the level of a liquid. The approach is based upon optical principles, and may be implemented entirely without the use of electrical sensing instrumentation (or, alternatively, electrical sensing instrumentation may be used). The approach is not sensitive to the presence of bubbles, fluid flows, or other interference from the liquid whose level is measured. Additionally, the system has no moving parts, is simple in structure and highly reliable, is of low cost and low weight, and has a low power consumption.
In accordance with the invention, a liquid-level sensor is operable with a volume in which a liquid may be present to different heights above a bottom of the volume. The liquid may be in a container or not in a container. The sensor comprises at least two solid optical conductors, preferably optical fibers. Each solid optical conductor includes an outer surface having at least one reflective surface discontinuity of sufficient size to interfere with a total internal reflection of the solid optical conductor when the reflective surface discontinuity does not contact the liquid. A support positions the reflective surface discontinuity of each of the at least two solid optical conductors at a location corresponding to a different height above the bottom of the volume. A light source introduces light into a first end of each of the solid optical conductors. A light detector structure receives light that has been introduced into each of the solid optical conductors and has traveled through the respective solid optical conductor at least as far as at least one of the reflective surface discontinuities of the respective solid optical conductor.
Most preferably, the light detector structure comprises a non-electrical structure. In one embodiment, the light detector structure comprises a light diffuser having a visual indication thereon of the liquid level. The light detector structure may instead be an electrical detector device.
In a most preferred embodiment, the light detector structure comprises a non-electrical light diffuser in the form of a light display. The light diffuser and display are positioned so that a second end of each of the solid optical conductors directs a respective output beam onto a respective region of the light diffuser. Each of the respective regions has a visual indication thereon of being illuminated by its respective output beam. Such visual indications may be, for example, a color of the illuminated region, alphanumeric characters on the illuminated region, and the like. The liquid level is indicated by the highest-value region that is illuminated.
The light detector structure is preferably positioned at a second end of each of the solid optical conductors. The light detector structure may instead be positioned at the first end of the solid optical conductor (the same end as the light source), and a second end of the solid optical conductor is made to be reflective of light.
The reflective surface discontinuity may be a transverse surface notch. It may instead be another type of non internally reflective portion of the outer surface of each of the solid optical conductor. An example is a non internally reflective flat or a sufficiently high-relief surface roughening on the outer surface of the solid optical conductors.
The present approach provides for liquid-level sensing using a number of optical conductors immersed into the liquid. The more optical conductors, the greater the resolution of the liquid level. In the preferred light-detector structure using a non-electrical, diffuser display, no electrical sensing device is utilized. The reliability of the system is thereby increased, although the distance between the liquid being sensed and the light-detector structure is limited. In many applications, however, there is no need for translating the liquid-level measurement into an electrical signal that may be transmitted over long distances. An example is the fuel level of an automobile. The fuel tank is typically located no more than about 15 feet or so from the driver""s instrumentation panel, which is well within the distance suitable for using the non-electrical sensing approach of the present invention. There is no electrical wiring at the point of the display, and there is no electromagnetic radiation at the display site. In other instances, both electrical and non-electrical sensing may be used together.
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment